1. Field of the Invention
The present invention relates to a method of applying a transfer voltage to each of a plurality of transfer members, which are opposed to image carriers carrying toner images, respectively, for transferring the toner images on the image carriers onto an image transfer target member to be moved through positions between the image carriers and the transfer members.
The invention also relates to an image forming apparatus, in which a plurality of transfer members are opposed to image carriers carrying toner images, respectively, and a transfer voltage is applied to each transfer member for transferring the toner images on the image carriers onto an image transfer target member to be moved through positions between the image carriers and the transfer members.
2. Description of the Background Art
An example of a structure of a color image forming apparatus in the prior art is schematically shown in FIG. 15.
An image forming apparatus Ap shown in FIG. 15 is an electrophotographic color image forming apparatus of the tandem type.
The image forming apparatus Ap includes four image forming units PC, PM, PY and PK. The image forming units PC, PM, PY and PR can form cyan, magenta, yellow and black images of toner on a record sheet, respectively.
The image forming units PC, PM, PY and PK are disposed at respective positions opposed to an outer peripheral surface of a sheet transporting belt 98 which can carry and transport the record sheet.
The image forming unit PC has a photosensitive member 91C. Around the photosensitive member 91C, there are arranged is a charger 92C, a laser exposing device 93C, a developing device 94C containing cyan toner, a transfer roller 95C, a cleaning device 96C and an eraser lamp 97C aligned in this order.
The image forming units PM, PY and PK likewise have photosensitive members 91M, 91Y and 91K, around which devices similar to those of the image forming unit PC are successively arranged, respectively.
In the image forming apparatus Ap, a color toner image based on the original image is formed on the record sheet in the following manner.
Toner images in cyan, magenta, yellow and black are formed on the photosensitive members 91C, 91M, 91Y and 91K, respectively, and then are successively transferred onto the record sheet R transported by the sheet transporting belt 98 so that the color image is formed on the record sheet R.
The toner images formed on the photosensitive members 91C, 91M, 91Y and 91K are electrostatically transferred onto the record sheet by applying transfer voltages onto the transfer rollers 95C, 95M, 95Y and 95K, respectively. In the transferring operation, the record sheet R is transported by the sheet transporting belt 98 through a path between the photosensitive members and the transfer rollers.
The transfer roller is supplied with the transfer voltage which is, e.g., a DC voltage. It is already proposed to use, as the transfer voltage, a sinusoidal AC voltage which is superimposed on a DC voltage, and therefore has a central voltage shifted from 0 volts. It is also proposed to apply, as the transfer voltage, a pulse AC voltage superimposed on a DC voltage.
However, when the toner images formed on the respective photosensitive members are transferred onto the record sheet by applying the transfer voltages to the transfer rollers, interfere of the transfer currents flowing through the record sheet may occur in the respective transfer regions, where the transfer rollers are opposed to the photosensitive members, respectively. In particular, if the record sheet is a paper sheet which has taken up moisture, the interference of the transfer current is likely to occur. When the interference of the transfer current occurs, transfer failure such as irregular transfer occurs so that good color toner images can not be formed on the record sheet.
The transfer failure due to the interference of the transfer current occurs not only in the image forming apparatus such as an image forming apparatus Ap, which is configured to transfer the toner images formed on the respective photosensitive members directly onto the record sheet, but also in an image forming apparatus which is configured to transfer temporarily the toner images formed on the respective photosensitive members onto an intermediate transfer member, and then transfer collectively all the transferred images onto the record sheet in a secondary transfer operation.